Aromatic polycarbonates belong to the group of industrial thermoplastics. They are distinguished by the combination of the technologically important properties of transparency, heat resistance and toughness. To obtain high molecular weight linear polycarbonates by the phase boundary process, the alkali metal salts of bisphenols are reacted with phosgene in the two-phase mixture. The molecular weight may be controlled by the amount of monophenols, such as e.g. phenol or tert-butylphenol. Practically exclusively linear polymers are formed in these reactions. This may be demonstrated by end group analysis.
Aromatic polycarbonates based on bisphenol A are also used in particular for the production of optical data carriers. However, they may also absorb up to 0.34 wt. % of water, which may have an adverse effect on the dimensional stability of data carrier materials. For other uses, in particular external uses, the hydrolysis again represents a certain problem.
U.S. Pat. No. 4,185,009, DE A 25 00 092 and JP B 79039040 describe a process in which, starting from mixtures of specific bisphenols with chain terminators and isatin-bisphenols as a branching agent, branched high molecular weight polycarbonates may be obtained after reaction with phosgene in a phase boundary reaction. DE A 42 40 313 describes copolycarbonates of improved flowability based on bisphenol A and bisphenol TMC with isatin-biscresol (IBC) as a branching agent.
DE A 19 913 533 describes highly branched polycarbonates, for the preparation of which oligomeric or polymeric branching agents are employed. DE A 19 943 642 describes branched polycarbonates which, because of their structural viscosity, are suitable for use as water bottle material.
U.S. Pat. No. 5,367,044 describes bottles molded of branched polycarbonate in which 1,1,1-tris-(4-hydroxyphenyl)ethane (THPE) is employed as a branching agent in amounts of 0.28–0.36 mol %.
Because of their better flowability compared with linear polycarbonates, branched polycarbonates are of particular interest for uses in which a good flow of the polymer melt at relatively high shear rates is desirable, i.e. for example in injection molding of complex structures. Branched polycarbonates are distinguished by structural viscosity and may no longer be regarded as Newtonian fluids.
However, these materials known from the prior art show unsatisfactory properties in respect of their resistance to hydrolysis and UV, attempts being made to improve these by expensive and costly additive measures.
A branched, ideally structurally viscous material which at the same time is distinguished by an improved resistance to hydrolysis and/or UV would therefore be desirable.
On the basis of the prior art, there has therefore for a long time been the object of discovering a material which has the typical advantages of polycarbonate as an industrial thermoplastic, but without having the disadvantages mentioned.
It has now been found, surprisingly, that certain branched polyformals and copolyformals are such materials.
Aromatic polyformals are also transparent thermoplastics which are built up from bisphenol units. In contrast to polycarbonates, however, the linking element between the bisphenol units consists not of carbonate units but include complete acetal units. While phosgene is the source of carbonate for the linking in polycarbonate, in polyformals e.g. methylene chloride or α,α-dichlorotoluene assumes the function of the source of the complete acetal linking element during the polycondensation. Polyformals may therefore also be regarded as polyacetals.
In contrast to polycarbonate, the preparation of aromatic polyformals may take place in a homogeneous phase from bisphenols and methylene chloride in the presence of alkali metal hydroxides. In this polycondensation methylene chloride simultaneously functions as a reactant and as a solvent. As in the polycondensation of polycarbonate, the molecular weight may also be controlled by small amounts of monophenol.
U.S. Pat. No. 4,374,974 already describes a process in which, starting from specific bisphenols, linear and cyclic oligo- and polyformals may be obtained after reaction with methylene chloride. A disadvantage of the materials which are to be obtained in this process is the relatively high content of cyclic reaction products, at several per cent, which has a very adverse effect on the mechanical properties. Furthermore, the polyformals described show very unfavorable swelling properties in organic solvents, as a result of which subsequent removal of the undesirable cyclic constituents is virtually impossible. Branched polyformals are not described in this specification.
DE A 27 38 962 and DE A 28 19 582 describe further and similar polyformals and their use as coatings or films, with the abovementioned disadvantages. Here also, the prior art teaches nothing of the preparation or use of branched polyformals or copolyformals.
EP A 0 277 627 describes polyformals based on specific bisphenols of the formula
and the possible use thereof as materials for optical instruments. In this Application the substitution of the bisphenols on the aryl radicals is described as necessary in order to force the optical anisotropy of the polyformals into a range suitable for optical uses. However, nothing is said about branched polyformals.
The linear polyformals already described in the prior art and their preparation processes are unsatisfactory in respect of their preparation or have, inter alia because of a high content of cyclic by-products, unsatisfactory mechanical properties, which manifest themselves e.g. in an increased brittleness. No preparation process for branched polyformals or copolyformals is described in the prior art. The prior art teaches nothing about the positive rheological properties of long-chain branched polyformals in combination with outstanding resistances to hydrolysis.
There was therefore the object of providing high molecular weight, branched polyformals and copolyformals and processes for their preparation which avoid these disadvantages. This object is achieved, surprisingly, by the use of specific bisphenols, methylene chloride and a trifunctional compound as a unit in homo- or copolyformals, and by the branched polyformals and/or copolyformals—herein (co)polyformals—according to the invention which are to be obtained by this means, and their preparation processes.
It has also been found here that the water uptake of the polyformals obtained has lower values than polycarbonates. Due to the favorable solution or swelling properties of the materials, cyclic impurities obtained may be separated off almost quantitatively and are then present only in the same order of magnitude as in the polycarbonate types which are nowadays usual. An impairment of the mechanical properties by cyclic impurities may be virtually ruled out as a result. It has moreover been found, surprisingly, that by suitable copolymer compositions glass transition temperatures of 130–170° C. may be realized, these being necessary for industrial use.
It was not to be expected that a hydrolysis-stable material which, due to its long-chain branching, at the same time has all the advantages of a structurally viscous polymer, such as e.g. easier processing in the range of relatively high shear rates or melt stiffness in the range of lower shear rates, may be produced in this manner.
For complete acetals, as these polymers indeed are to be considered, these polyformals completely unexpectedly also show in fact an extreme hydrolytic stability at higher temperatures both in an alkaline and in an acid medium. It is moreover found that the polymers themselves are considerably more stable compared with polycarbonate in the boiling test in water.